Marriage is no longer what it once was. Since the 1970s, and accelerating in recent decades, the link between formal marriage and family life has weakened dramatically. Nearly half of American adults are now unmarried at any given time, and two of five children are born to unmarried parents. At the same time, delayed marriage, divorce and remarriage, and changing gender roles have transformed the content of marriage itself. Despite these changes, the federal income tax and the Social Security system continue to define “family” based on formal marriage, and our casebooks teach students that the economic vulnerability of the married woman is the central problem of gender in welfare-state design.